1. Field of the Invention
The present invention relates to a camera system which includes an image-taking apparatus, such as a television camera and a video camera, and a lens apparatus mounted on the image-taking apparatus.
2. Description of the Related Art
Camera systems for use in television broadcast or video recording include an auto-focus (hereinafter abbreviated as AF) type in which a video camera is integral with a lens and focusing is performed automatically, and a manual focus (hereinafter abbreviated as MF) type in which a lens is removably mounted on a camera and focusing control is performed in response to manual operation of a manipulation member.
Recently, the AF has also been used in camera systems which employ interchangeable lenses.
Conventionally, only AF-capable lenses can be used in AF camera systems and only MF-capable lenses can be used in MF camera systems. In recent years, however, there is an increasing need for camera systems in which both of MF-capable lenses and AF-capable interchangeable lenses can be used.
In the MF-capable lenses, however, a so-called front focus type is typically employed in which a focus lens is placed closer to an object side than a variable magnification lens. On the other hand, in the AF-capable lenses, a so-called rear focus type is usually employed in which a focus lens is placed closer to an image side than a variable magnification lens. This is because of the operability, controllability and portability of the lenses.
Japanese Patent Application Laid-Open No. H6 (1994)-62305 has disclosed an AF method in such a camera system, for example. In the AF method, a signal indicating the sharpness evaluation value of an object is extracted from a video signal taken by a camera to move a focus lens in a direction in which the strength of the signal is increased. The AF method is called a climbing method (a contrast detection method).
With a smaller size and a lighter weight of the entire camera system, zoom lenses of the rear focus type allowing reductions in size and weight are used in the AF-capable lenses. In the zoom lenses of the rear focus type, the position of an image plane changes with varied magnification even when the same distance to an object is maintained, so that the position of a focus lens needs to be adjusted in association with the varied magnification. Such a method of adjusting a focus lens has been disclosed, for example, in Japanese Patent Application Laid-Open No. H1 (1989)-280709, Japanese Patent Application Laid-Open No. H8 (1996)-220414 and the like.
Description is now made for the structure of a conventional zoom lens of the rear focus type. In FIG. 4, the zoom lens is comprised of four lens units including, in order from an object side, a fixed front lens unit (a first lens unit) 131, a variable magnification lens unit 132 which is movable on an optical axis (a second lens unit, hereinafter referred to as a zoom lens unit), a fixed lens unit (a third lens unit) 136, and a focus lens unit (a fourth lens unit) 118 which is movable on the optical axis for correcting image plane variations in varying magnification and for focusing.
Reference numeral 111 shows a zoom ring. When the zoom ring 111 is rotated, the zoom lens unit 132 is moved in the optical axis direction by a cam (not shown) formed on a fixed barrel 137 to perform variable magnification.
Reference numeral 115 shows a zoom motor which drives the zoom ring 111 through interlocking gears 133 and 134. Reference numerals 119 and 120 show guide bars which guide a moving frame 122 for holding the focus lens unit 118 in the optical axis direction.
Reference numeral 121 shows a focus motor which is realized by a step motor, for example. The moving frame 122 has a screw portion formed thereon which engages with a screw shaft 123 which is driven by the focus motor 121. Thus, when the screw shaft 123 is rotated by the focus motor 121, the moving frame 122 is moved in the optical axis direction together with the focus lens unit 118. Reference numeral 135 shows a stop unit which adjusts an amount of light.
FIG. 6 shows trajectory data which represents the positional relationship between the zoom lens unit 132 and the focus lens unit 118 on the optical axis for various object distances. The trajectory data shows the moving trajectories of the focus lens unit 118 for maintaining an in-focus state of an object at distances from INF (infinity) to MOD (minimum object distance).
For the rear focus lens, the focus lens unit 118 exists closer to an image side than the zoom lens unit 132, so that simply driving the zoom lens unit 132 in the structure shown in FIG. 4 causes the position of the image plane to be changed with varied magnification. To maintain the in-focus state, it is necessary to determine the position where the focus lens unit 118 should be placed from the position of the zoom lens unit 132 and the object distance in the trajectory data of FIG. 6 to drive the focus lens unit 118 in association with varied magnification.
Next, description is made for AF processing in the aforementioned camera system which employs the rear focus lens with reference to a block diagram of FIG. 5. In FIG. 5, reference numeral 102 shows an image-pickup element such as a CCD sensor and a CMOS sensor, and 103 a camera processing circuit which produces a video signal based on an output signal from the image-pickup element 102. The video signal is output to an AF circuit 104. The AF circuit 104 extracts a high-frequency component from the video signal and outputs it as an AF evaluation value signal to a CPU 140.
In normal AF processing (AF processing without varying magnification), the CPU 140 causes the focus lens unit 118 to wobble through a lens control section 139 on the zoom lens side to determine a drive direction of the focus lens unit 118 in which the strength of the AF evaluation value signal is increased. The CPU 140 outputs a focus control signal for driving the focus lens unit 118 by a predetermined drive amount in the determined direction and drives the focus lens unit 118 until the AF evaluation value signal is at the maximum.
In the AF processing with varied magnification, the CPU 140 calculates the drive amount of the focus lens unit 118 by using the trajectory data in FIG. 6 stored in a trajectory data memory 145 and information about the current positions of the zoom lens unit 132 and the focus lens unit 118 detected as described later and outputs a focus control signal according to the calculation result.
On the zoom lens side, reference numeral 106 shows a zoom position detector which detects the position of the zoom lens unit 132 on the optical axis. Reference numeral 107 shows a zoom motor driver which drives the zoom motor 115. Reference numeral 125 shows a focus motor driver which drives the focus motor 121 realized by the step motor. The number of drive pulses input to the focus motor 121 is counted by the CPU 140. The count is used to detect the position of the focus lens unit 118.
Reference numeral 138 shows a stop position detector which detects the position of blades (that is, a stop value) provided for the stop unit 135.
The lens control section 139 produces a drive signal for the zoom lens unit 132 based on a zoom control signal produced in a zoom operation circuit 141 in response to manipulation of a zoom switch 142 such as a seesaw switch. The lens control section 139 also produces a drive signal for the focus lens unit 118 based on a focus control signal output from the CPU 140.
The trajectory data shown in FIG. 6 depends on optical characteristics of a zoom lens. For example, the moving amount of a zoom lens unit varies according to the magnification of a lens. For this reason, if interchangeable lenses having various optical characteristics are used for one camera, the trajectory data of each interchangeable lens is transmitted to the camera for storage in the trajectory data memory 145 when each interchangeable lens is mounted on the camera.
The focus control signal produced in the camera system which has the AF-capable lens of the rear focus lens type includes a signal which represents the drive amount of the focus lens unit 118 calculated by using the current positions of the zoom lens unit 132 and the focus lens unit 118 and the trajectory data in order to correct the image plane variations in varying magnification.
In contrast, in the camera system which has the MF-capable lens of the front focus type, the focus lens is placed closer to the object side than the variable magnification lens and thus the position of the image plane is not changed with varied magnification. This eliminates the need to drive the focus lens unit in varying magnification.
Now, description is made for the structure of a conventional front focus type zoom lens and a manual focus control system. In FIG. 7, on the side of a camera 201, reference numeral 110 shows a zoom/focus operation circuit. Connected to the zoom/focus operation circuit 110 are a zoom operation member 112 which outputs an operation signal for servo-driving a zoom lens unit (not shown) and a focus operation member 113 which outputs an operation signal for servo-driving a focus lens unit (not shown). The zoom/focus operation circuit 110 produces and outputs control signals according to operation amounts of the respective operation members. Reference numeral 144 is a camera control section which is responsible for control of the camera.
On the other hand, on the zoom lens side, reference numeral 114 shows a focus motor which drives the focus lens unit through an interlocking gear (which has no reference numeral). Reference numeral 116 shows a focus motor driver which drives the focus motor 114 in response to a focus drive signal from a lens control section 143.
Reference numeral 117 shows a focus position detector which detects the current position of the focus lens unit 118 and transmits it to the lens control section 143. Reference numeral 124 shows a focus ring associated with the focus lens unit. The focus ring 124 is rotated by the focus motor 114 to move the focus lens unit, thereby performing manual focusing.
Reference numeral 143 shows the lens control section which controls the zoom motor 115 and the focus motor 114.
When the focus operation member 113 is operated, an operation signal output from the focus operation member 113 is input to the lens control section 143 as a focus control signal through the zoom/focus operation circuit 110. Similarly, when the zoom operation member 112 is operated, an operation signal output from the zoom operation member 112 is input to the lens control section 143 as a zoom control signal through the zoom/focus operation circuit 110.
These control signals are converted by the lens control section 143 into a focus drive signal and a zoom drive signal for achieving drive of the motors and output to the drives 116 and 107, respectively. In response to the drive signals, the drivers 116 and 107 drive the focus motor 114 and the zoom motor 115 to rotate the focus ring 124 and a zoom ring 111, respectively. In this manner, the focus lens unit and the zoom lens unit are driven.
A controller 109 which can be used to perform zoom and focus operation is connected to the camera for use in such a system, besides the operation members 112 and 113. The controller 109 outputs a control signal according to the amount of operation thereof, and the control signal is output to the lens control section 143 through the camera control section 144.
As described above, when the rear focus lens is used to perform the AF control, the focus control signal output from the camera for correcting the image plane variations associated with varied magnification is a signal which represents the position to which the focus lens unit should be moved (the drive amount), determined from the trajectory data in FIG. 6, the current position of the zoom lens unit, the current position of the focus lens, and the object distance.
On the other hand, when the front focus lens is used to perform the MF control, the focus control signal output from the camera is a signal produced according to the operation amount of the focus operation member 113.
In other words, the AF camera system with the rear focus lens and the MF camera system with the front focus lens produce the focus control signals on the camera sides in different manners, so that only dedicated interchangeable lenses can be used in each of the systems.
Next, description is made for a conventional camera system in which both of an MF-capable lens and an AF-capable lens can be used for one camera with reference to a block diagram of FIG. 8. In FIG. 8, components identical to those in FIGS. 5 and 7 are designated with the same reference numerals as those in FIGS. 5 and 7 and description thereof is omitted.
In FIG. 8, reference numeral 200 shows a lens control section which is responsible for control of a zoom lens. The zoom lens is an MF-capable interchangeable lens or an AF-capable interchangeable lens.
When the zoom lens is mounted on a camera 301, a camera control section 305 provided in the camera 301 outputs a request for transmission of an identification signal for determining whether the zoom lens is an MF-capable lens or an AF-capable lens. In response to the request, the lens control section 200 transmits the identification signal back to the camera control section 305. The camera control section 305 switches between methods of producing a focus control signal depending on the determination result. In this manner, the focus control signal appropriate for the mounted lens is produced and output to the lens.
It should be noted that some zoom lenses have no identification signals to be transmitted in response to the transmission request from the camera control section 305, in which case an identification signal cannot be transmitted back. In this case, the camera control section 305 determines that no reply is made in a certain time period and considers the mounted lens as an MF-capable lens before it starts control.
The control of the MF-capable lens is performed as described above with reference to FIG. 7. The control of the AF-capable lens is performed as described above with reference to FIG. 5. When the AF-capable lens is mounted, the camera 101 takes the trajectory data shown in FIG. 6 from the lens side and stores it in the trajectory data memory 145.
As described above, in the conventional camera system, various types of lenses are used with different arrangements such as the front focus lens and the rear focus lens and with different focus control methods such as the MF control method and the AF control method. In a camera which can accept only a particular type of lens, if another type of lens is mounted thereon, normal focus control cannot be performed.
Specifically, since the focus control signal to be provided for the MF-capable front focus lens by the camera is not consistent with the focus control signal to be provided for the AF-capable rear focus lens, both types of lenses cannot be used in each of the MF-capable camera and the AF-capable camera.
As described in FIG. 8, it is possible that the type of the mounted lens is determined on the camera side to switch between the methods of producing the focus control signal. However, to produce the focus control signal appropriate for the AF-capable lens (the rear focus lens) on the camera side, the camera must take the large amount of trajectory data from the lens at power-up, and the camera needs to have a memory of large capacity for storing the trajectory data and a calculation function for producing the focus control signal suitable for the AF-capable lens.